a
Table 1. Heck Coupling Catalyzed by Sol-Gel Entrapped PdCl2(PPh3)2
entry
haloarene
unsaturated substrate
time (h)
T (°C)
products (yield, %)b,c
1
2
3
4
5
6
PhI
PhI
PhI
PhI
PhI
4-O2NC6H4Br
PhCHdCH2
12
12
12
8
12
16
110
110
110
90
90
110
PhCHdCHPh (E, 84; Z, 5)
4-C6H4CHdCHPh (E, 41)
PhCHdCHCO2Me (E, 86)
PhCHdCHCN (E, 39; Z,30)d
PhCtCPh (48)d,e
4-ClC6HCHdCH2
MeOCOCHdCH2
NCCHdCH2
PhCtCH
PhCHdCH2
4-O2NC6H4CHdCHPh (E, 28)
a Reaction conditions: 4 mmol of haloarene, 4 mmol of unsaturated substrate, 5 mmol of Pr3N, catalyst containing 0.0427 mmol of PdCl2(PPh3)2 in 15
mL of toluene. b In entries 1-4 and 6, the missing percentages reflect the unreacted starting materials. c The NMR, IR, and MS of the products were found
to be identical with those of authentic samples either obtained from commerical sources or prepared according to literature procedures.17 d Contaminated
with some polymeric material. e Yield in the third run.
palladium complexes on silicas of various porosity and
surface areas, as well as on supported liquid-phase catalysts.13
One approach to prepare supported leach-proof catalysts for
the Heck reaction is based on the application of chemically
bound palladium compounds to functionalized porous silica.14
Indeed, some of these variations in the Heck reaction proved
to give high yields and to permit recycling of the catalysts.
However, the preparation of most of these heterogenized
catalysts is asociated with tedious procedures.
In this study, we have employed for the first time a sol-
gel physically entrapped version of the PdCl2(PPh3)2 catalyst
for the Heck coupling process. This heterogeneous catalyst
is characterized by its extremely simple preparation.15 It is
a perfectly leach proof, highly porous material (typical N2-
BET surface areas of 490 m2 g-1, and average pore diameters
of 27 Å have been recorded) and completely air stable for
many months.
When, for example, a solution of 417 mg (4 mmol) of
freshly distilled styrene, 816 mg (4 mmol) of iodobenzene,
and 1.14 mL (5 mmol) of tripropylamine in 15 mL of toluene
was refluxed for 12 h in the presence of a sol-gel
encapsulated catalyst containing 30 mg (0.0427 mmol) of
PdCl2(PPh3)2, the resulting solution consisted, after removal
of the catalyst, of 84 and 5% trans- and cis-stilbene,
respectively, as well as 10% unreacted starting materials.
Column chromatography of several experiments afforded
77-82% of the products. The filtered catalyst was treated
with 15 mL of boiling CH2Cl2 for 10 min followed by 20
min of sonication with the same solvent, dried briefly at 1
mm, and reused. Four runs of 12 h indicated that the
immobilized catalyst does not lose any catalytic activity upon
recycling.
summarized in Table 1. Whereas iodobenzene couples
smoothly with styrenes and acrylic acid derivatives, bro-
mobenzene reacts very slowly (∼2% after 20 h). It can,
however, be activated by introduction of an electron-
attracting group into the aromatic ring (see, e.g., entry 6).
The arylation products of acrylonitrile and of phenylacetylene
in boiling toluene were accompanied with some polymeri-
zation products. The polymerization could however be
decreased significantly by lowering the reaction temperature
to 90 °C. Upon recycling of the immobilized catalyst, the
yields were hardly affected in all cases tested except in the
arylation of phenylacetylene. Under the conditions given in
Table 1, the yields of isolated tolan in the first three runs of
12 h were 25, 30, and 48%, respectively. Such an increase
in rate has already been observed in our previous studies
and is attributed to slow modification and activation of the
entrapped palladium compound in the presence of alkynes.16
We have already shown that the entrapment of catalysts
within sol-gel matrixes not only permits facile recycling of
the dopant but also protects it from “hostile” and opposing
chemicals. This enabled us to conduct multistep one-pot
reactions with entrapped catalysts in the presence of (sepa-
rately) entrapped catalyst poisons18 and with acidic catalysts
in the presence of bases,19 to perform simultaneously
oxidation and catalytic hydrogenation,20 and to carry out
esterfications by sol-gel entrapped lipase in the presence
of an enzyme destroying metallic catalyst.21 Now we have
investigated the possibility to couple the Heck vinylation by
entrapped PdCl2(PPh3)2 with a photochemical process in the
same reaction flask.
Photocyclization of stilbenes is well-known,22 and the
combination of a Heck vinylation with photolysis in two
separate steps has been employed in the syntheses of various
Some further Heck coupling experiments by the sol-gel-
entrapped PdCl2(PPh3)2 have been conducted. The results are
(16) Cf. the gradual activation of another sol-gel entrapped catalyst
during hydrogenation reactions: Blum, J.; Gelman, F.; Abu-Reziq, R.;
Miloslavski, I.; Schumann, H.; Avnir, D. Polyhedron 2000, 19, 509.
(17) (a) Melpolder, J. B.; Heck, R. F. J. Org. Chem. 1976, 41, 265. (b)
Breuer, E.; Bannet, D. M. Tetrahedron 1978, 34, 997. (c) Heck, R. F. Org.
React. 1982, 27, 345.
(18) (a) Gelman, F.; Blum, J.; Avnir, D. J. Am. Chem. Soc. 2000, 122,
11999. (b) Gelman, F.; Blum, J.; Schumann, H.; Avnir, D. J. Sol-Gel Sci.
Tech. 2003, 26, 43.
(19) Gelman, F.; Blum, J.; Avnir, D. Angew. Chem., Int. Ed. 2002, 40,
3647.
(20) Gelman, F.; Blum, J.; Avnir, D. New J. Chem. 2003, 27, 205.
(21) Gelman, F.; Blum, J.; Avnir, D. J. Am. Chem. Soc. 2002, 124, 14460.
(22) Mallory, F. B.; Mallory, C. W. Org. React. 1984, 30, 1.
(13) See, e.g.: Fujita, S.-I.; Yoshida, T.; Bhanage, B. M.; Arai, M. J.
Mol. Catal. A: Chem. 2002, 188, 37.
(14) Clark, J. H.; Macquarrie, D. J.; Mubofu, E. B. Green Chem. 2000,
2, 53.
(15) Typically, 5 mL (33.9 mmol) of tetramethoxysilane (TMOS) was
hydrolyzed by stirring for 1 h at 25 °C with 4 mL (222 mmol, r ) 6.7) of
triply distilled water (TDW). A solution of 30 mg (4.27 × 10-2 mmol) of
PdCl2(PPh3)2 in 11 mL of THF was added. Addition of 50 µL of an 0.1 M
solution of tetrabutylammonium fluoride induced gelation of the reaction
mixture within 5-8 min. After being aged for 16 h, the solid catalyst was
treated for 1 h with boiling toluene, dried for 10 h at 1 mm, sonicated, and
refluxed for 30 min with 15 mL of CH2Cl2 and redried for 3 h at 1 mm to
give 2.3 g of the entrapped catalyst.
926
Org. Lett., Vol. 6, No. 6, 2004